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The monstrous in the magnificent

He was a court painter who dared to anger his royal patron - an intellectual, wedded to ambiguity, who could show the public mask and the person behind it. Jonathan Jones examines the work of Hans Holbein, perhaps the greatest, but certainly the truest, of portraitists

It was an age of discovery, and strange reports from strange places were being received every day. Monsters are no longer news, joked Thomas More in his parody of travel-writing, Utopia.

They were no longer fiction, either. The fantastic dragons of the medieval imagination were being displaced, in the Renaissance, by outlandish creatures and exotic peoples that actually existed, seen by explorers or brought back to Europe.

The Portuguese fetched a rhinoceros out of Africa to Lisbon; after reading about it, and in a spirit of factual curiosity, the German artist Albrecht Dürer made his print The Rhinoceros, a portrait of a living, material monster, a singularity, with its horn and its beak-like face and its knobbly, armoured hide.

With the same sense of wonder, of observing a thing unique on earth, Dürer's countryman, Hans Holbein, portrayed another immense monster: Henry VIII, king of England.

Holbein's painting of Henry, with his father, mother (both long since dead) and third wife, Jane Seymour, done in 1537 on a wall of Whitehall Palace, was destroyed in 1698 in the fire that consumed the entire palace except for Inigo Jones's 17th-century Banqueting House, and a few fragments lying under the Ministry of Defence or behind Downing Street's security barriers.

And yet this is the most influential, and efficacious, royal portrait in British history. As soon as it was finished, it was copied, imitated, quoted - by Holbein himself, among others; an instant icon. And Henry VIII is remembered, still, as he has been for five centuries, as a huge, hearty man, chest broad as a wall, his legs arrogantly apart and his elbows flaunting his right to push everyone aside.

You can still see a trace of Holbein's colossus. The left third of the cartoon for the Whitehall Palace painting - a life-sized drawing done in ink and watercolour on many small sheets of paper - survives in the National Portrait Gallery. Henry's eyes resemble a sperm whale's, tiny holes in a mountainous head.

There is no avoiding the grotesque freakishness of this king. This prodigy. Two years or so before Holbein made his drawing, the French ex-monk François Rabelais published Gargantua, a hilarious, bawdy book about a giant and his appetites. Henry is Gargantuan. Stand up close and he dwarfs you. His bulging codpiece, a dome pushing his clothes apart as if it cannot be contained, is level with your eyes. His dagger, curved like a boar's penis, is held in his left hand just below the codpiece. His chest is a sail of impregnable flesh. All Henry's weight, in Holbein's vision, becomes width. His shoulders extend a foot each or more on either side of his neck. His neck is thicker than his head.

Holbein's Henry, sensual and energetic, is the Renaissance made flesh. Behind him lurks the grim, emaciated personification of the middle ages, the past, Old Mortality: his father. Literally pushed back into the shadows by his son, the man who established the Tudor dynasty when he won the Battle of Bosworth acts as a posthumous foil to his successor.

Henry VIII is fat, Henry VII is thin. Henry VIII fully inhabits his clothes, Henry VII shrivels inside his. Henry VIII glares confidently out of the picture, Henry VII is lost in wan contemplation. The ruling monarch is vital, keen, his father a flimsy, depressive shade.

Holbein's Henry VIII is Carnival, triumphing over Lent. He is the Lord of Misrule and he is Hercules. Whatever he is, he is not quite human. Hans Holbein the Younger single-handedly made Henry VIII and his court the first British people we can picture clearly in our minds.

We owe the image of the rampant king to this enigmatic German, born to an artist father in Augsburg in 1497 or 1498, trained in Basel, who came to Britain looking for work, who ended up as the salaried court artist of Henry VIII and who, just as he was planning to go home to Basel, rich and famous, contracted plague and died here in the autumn of 1543.

And yet the more you look at Holbein's Whitehall cartoon, the more complex it becomes. It is not, after all, a realistic portrait, but a piece of philosophy, a painting that invites us to think about life, death, power and the limits of the human. Holbein is not simply a reporter, but a Renaissance artist given to obscure and melancholy thought.

Holbein's portraits have excessive presence. They are quite simply among the greatest portraits that exist, and in verisimilitude have no rivals. They are startling, unforgettable images, with the precision of living, breathing waxworks. The Pygmalion-like Holbein had uncanny gifts.

He was able, building on the research of Renaissance artists into perspective (his predecessor Dürer wrote a book about it), to give his people the illusion of three dimensions. They are solid, fleshy presences, existing in tangible space. Yet because he is so dead-on, so perfect, because his people seem to stand so naturally before us, we are in danger of missing the intellectualism of Holbein's art.

Holbein was accepted in his own lifetime as Britain's visual historian. He still is. The National Portrait Gallery's early Tudor section is full of copies of Holbein cherished as history rather than art. Most copied of all was his portrait of Thomas More - humanist, social radical, persecutor of Protestants and canonised martyr - with his family: his father, wife, children and his fool, Henry Patenson. Although the portrait is lost, it is known through several 16th-century imitations, as well as a surviving sketch.

The sketch is a stunning encounter with the legendary dead. Thomas More looks so alive, so real, so characterful, as do the women sitting with open books - lots of books; a family that loves to read. And yet it is anything but straightforward. It is an essay, a pictorial equivalent of something More might have written.

Disturbingly, a clock is over all their heads, ticking away, recording the approach of death. The books are an image of time well spent. The leisure of this family is nothing like the idle hunting and jousting that took up time at court ("Hunt, sing and dance/My heart is set/All goodly sport," goes a song written by Henry VIII).

This intimate family portrait is, in fact, a clue to the intellectualism of Hans Holbein the Younger. When he first came to England in 1526-8, Holbein brought a letter of introduction to Thomas More; he had another addressed to Peter Gillis in Antwerp, the friend at whose house More claims, in Utopia, to have met a sailor with tales of just societies. Both letters of introduction were written by Desiderius Erasmus, theologian, wit and the greatest intellectual in northern Europe.

Erasmus's masterpiece is a radical Renaissance defence of the comic, irrational and ambiguous, Praise Of Folly, written to entertain Thomas More, while staying at his house. Its title, Moriae Encomium, is a pun on his friend's name.

When More and Erasmus joked, they joked in Greek and Latin. Praise Of Folly is a eulogy delivered by Folly to herself - a parody of the formal speeches delivered by rhetoricians in Renaissance Europe. At the same time it seems to be serious, to speak truths, and this is Erasmus's argument: that truth can be foolish, folly truth; that everything is two-sided and it is only in a tolerant, ecstatic, irrational love that truth can be discovered.

The key image of Praise Of Folly is the "Silenus", named after the ancient Greek tutor of Bacchus, god of wine. A Silenus is a box that is grotesque and ridiculous on the outside but contains wonderful things; "so that what is death at first sight, as they say, is life if you look within, and vice versa, life is death. The same applies to beauty and ugliness, riches and poverty, obscurity and fame, learning and ignorance, strength and weakness, the noble and the base-born, happy and sad... "

Praise Of Folly, published in 1511, set imaginations on fire. Rabelais reused the image in the preface to his outrageous carnival of a book, defending his vulgar text as a Silenus containing truth. And so, I think, did Holbein.

Holbein got it from the horse's mouth. Erasmus came to live in Basel, where Holbein began his life as an artist, in 1521. Holbein painted several portraits of the great man; before this, he illustrated a copy of Praise Of Folly.

In Holbein's portrait of Erasmus at the National Gallery, the benignly smiling scholar, with books on a shelf behind him, is flanked by a florid piece of grotesque carving; like the exterior of the Silenus in Praise Of Folly. And this image of double-sided truth suffuses Holbein's paintings. His two masterpieces - The Ambassadors, in London, and Christ Dead In The Grave, in Basel - I believe specifically illustrate Erasmus's thesis that life and death are each contained in the other.

The Ambassadors is a distillation of the Renaissance. Everything is in it. On a table between Jean de Dinteville and Georges de Selve, representatives of the French king in London, are emblems of exploration, learning and trade: a globe that shows Brazil, mathematical and time-telling instruments, music, a carpet - used as a tablecloth - imported from Turkey.

The men are reserved, po-faced; they give little away. Behind them is a claustrophobic green curtain that presses the picture forward, slightly parted to reveal Christ on the cross. A distorted, at first baffling, black and white form smears itself in a disruptive oval across the painting; viewed from the right-hand side, it reveals itself as a human skull.

"Vice versa, life is death." There have been many explanations of this deliberately confounding picture, but the simplest is offered by Erasmus. Life is death: all the material and intellectual and artistic energies of the Renaissance, recorded in this picture, all the politics - represented by the Ambassadors themselves - and the pastimes can switch in a second to their opposite: death.

This Silenus inversion is, historically, the literal truth of the painting. If we were to dig up these two men, skulls are about all we would find, if not mere dust. Holbein is pointing out the grim truth about all the historical images of ancient Romans that were revered in Renaissance Europe - at Henry VIII's Hampton Court, terracotta portraits of Roman emperors remind us of past greatness. But in truth, Holbein points out, the reality of greatness, in time, is a skull. Life is death.

Holbein's portrait of Christ in the tomb - the most radically realistic Renaissance image of the entombed Christ, beard stuck in the air, limbs stiff and sinews hardened, much deader than Mantegna's dead Christ, the absolute horrible fact of death, its essence - is the reverse of the Silenus. This Christ could not be more dead, but he will be resurrected; "What is death at first sight... is life if you look within." Christ is portrayed inside his coffin, inside a hideous, and in this case deceptive, box - a Silenus.

But it wasn't just the reversible polarity of life and death that fascinated Holbein. His paintings explore another ambiguity cited by Erasmus: "beauty and ugliness". Holbein is very different from German contemporaries such as Lucas Cranach, whose art is fearsomely fleshy and gothic.

Holbein, like Dürer, believed in the Italian Renaissance with its classical ideal of beauty; he may have visited Italy, and he definitely saw Italian paintings, probably including portraits by Leonardo da Vinci, in France. But Holbein's portraits don't slavishly copy an ideal of beauty. Instead, he puts this ideal in juxtaposition with complex reality; he poses it as a question. What is beauty, and what is ugliness? And doesn't all beauty have something ugly in it?

Holbein posed the question of beauty versus ugliness most radically, honestly and dangerously - endangering himself, risking his neck - when Henry VIII gave him the most bizarre mission that ever fell to a court artist. It happened in 1538, following the death of Jane Seymour, after she gave birth to Edward, Prince of Wales, whom Holbein portrayed as a miraculous, divine infant in man's clothing with his hand raised in podgy benediction.

The death of Henry's third wife left the Bluebeard monarch on the prowl again. Henry wanted another bride, but eligible women were getting thin on the ground: his reputation preceded him. So Holbein was dispatched to portray likely candidates among the politically useful dynasties of Europe.

He met Christina of Denmark, the 16-year-old, recently widowed Duchess of Milan, in Brussels; she posed for him for three hours. When Christina wisely spurned her murderous royal suitor, Holbein was sent to portray Anne of Cleves. His portrait convinced Henry - who had been unimpressed by other artists' images of Anne - that she was the one.

But when the king sneaked a look at his bride on the eve of their marriage, he was bitterly disappointed and claimed he couldn't get an erection for this "fat Flanders mare". The marriage was dissolved on grounds of non-consummation; Thomas Cromwell, Henry's Lord Great Chamberlain, an earthy Holbein portrait subject, who had encouraged the marriage for strategic reasons, got his head chopped off; Holbein lost Henry's favour and never received another royal commission on the scale of the Whitehall mural.

Holbein's portraits of Anne of Cleves and Christina of Denmark both survive; and they reveal that Holbein never exactly lied to Henry. Neither Christina nor Anne is portrayed as some unreal Renaissance beauty; neither is a Botticelli. In both portraits, Holbein wonders aloud what beauty is; he asks the cunning, playful question posed by Erasmus.

The miniature portrait of Anne of Cleves that Holbein made from his drawing of her, now in the V&A, is shrunken but perfect, a symmetrical composition of gilded triangles on a blue background inside a circular box carved from a single piece of elephant tusk, 15cm in diameter, with a removable lid in the shape of a densely ornamental Tudor rose.

Anne is a princess bottled, so you can put her in a pouch and take her out to examine her from time to time. Along with his contemporary Lucas Hornebolte, Holbein established miniature portraiture as a peculiarly English art: the epitome of the cult of the portrait in 16th- and 17th-century Britain.

And yet the face that is the whole point of this exercise is an absence, characterless and vacant; Holbein doesn't make Anne of Cleves look ugly, but he makes her look dull. She looks straight out at us, but her eyes don't meet ours. Holbein is playing an Erasmian joke, a joke that obviously went right over Henry's head.

Anne of Cleves isn't beautiful - she just wears beautiful clothes. Her ornate dress and jewels constitute a Silenus in reverse; beautiful on the outside, ugly within. This Erasmian joke is made explicit by the ivory box: it is a beautifully carved container, the opposite of a Silenus - a warning, to anyone who had read Praise Of Folly, that its contents might be meagre.

With the portrait of Christina of Denmark that Holbein made for Henry, today in the National Gallery, he did the opposite. Christina stands lifesize inside a tall, narrow painting, the shape of a Tudor coffin lid. She is sealed inside black, smothering mourning clothes: an ugly Silenus exterior.

The 16-year-old Christina is so modestly covered up that it seems impossible, putting yourself in the position of Henry VIII, to decide if she is beautiful or ugly. Christina's white hands and face stand out, smooth and chubby, from the darkness. She has something almost infantile about her; and in an adult almost ugly.

Perhaps it's because you can't see any of her hair under her round, black cap that she resembles a big baby. Her face is harmonious, but not in an Italian, classical way; it's an inverted pear, with big almond eyes; and then her strong, red lips, the fascinating centre of the picture, closed in a half-smile that is reminiscent of the Mona Lisa.

Christina is charismatic and vibrantly powerful, a woman with character, a tough and reserved presence: it is impossible to tell if this woman is "beautiful" or "ugly". She is a smiling Silenus: her beauty is on the inside. Henry fell for the picture - he kept this painting in place of the woman who evaded his gross embrace.

History, documentary, evidence - we get all this from Holbein's paintings. We are right to feel that he transports us into the past; right to trust him, even if Henry came a cropper by doing so. But the art of Hans Holbein the Younger is not innocent. Complexity, secrecy, double meanings are what he loves best.

In 16th-century Europe, the idea of the portrait was anything but simplistic or trusting. It was an age of suspicion. Courtiers practised a pleasing mask to present to their prince. Princes were encouraged, by Machiavelli, to lie to their subjects. Religious division meant that dissimulating your faith might be essential to stay alive.

A painting that dates from about 1510, and is today attributed to Ridolfo del Ghirlandaio, in the Uffizi in Florence, makes this anxiety explicit. The portrait is of an unnamed woman, sad, enigmatic; it has been variously called The Veiled Woman and The Nun.

But even more mysteriously, it comes with a "cover", a painting designed to slip over it and conceal it. This false panel is painted with grotesque reliefs including, disturbingly, a humanoid, flesh-coloured mask with tight lips and black, empty eye-holes. In Latin, a painted inscription reads Sua cuique persona - "To each his own mask."

To each his own mask. This is a long way from a simple notion of identity, of depiction. We present masks to the world and a portrait is a record not of a self, but a mask, a performance, in which subject and artist conspire. It is a fiction. It is not to be taken at face value.

Looking at the Whitehall portrait of Henry VIII, you realise Holbein is praising the king as a monster, a figure of generous, excessive, superabundant force of life. This is an Erasmian joke: Henry is a prodigious image of folly, the folly that makes him a great and generous king. But then, the Silenus is double-sided.

To praise someone as a monster is also to call him a monster. We think of Henry like this, we imagine him like this. Character: he has it in abundance. He's one of those people who is always theatrically himself. His public self, like a blubbery suit he puts on every morning before stepping out of his private chamber, is a disguise that has become the man: a mask stuck to his face. Henry VIII is, as Holbein records, unique. But you wonder if he ever wanted to be someone else, a private subject without so much blood to account for.

For Erasmus, by the logic of the Silenus, every appearance is deceptive. A king without virtue is really a beggar: "We all agree a king is rich and powerful, but if he lacks all spiritual goods and can never be satisfied, then he's surely the poorest of men." Holbein's huge Henry is, as you look at him, not only vast but strangely thin. He is flat, as flat as the paper he is drawn on.

· An exhibition, Hans Holbein 1497/8-1543, is at the Mauritshuis, the Hague, Holland, from today until November 16 (details on hansholbein.nl). The accompanying catalogue, Hans Holbein The Younger: Painter At The Court Of Henry VIII, by Jochen Sander and Stephanie Buck, is published by Thames & Hudson on September 1 at £32. To order a copy for £29, plus UK p&p, call 0870 066 7979.